# std::ranges::fold_right_last

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::bidirectional_iterator I, std::sentinel_for<I> S, /*indirectly-binary-right-foldable*/<std::iter_value_t<I>, I> F > requires std::constructible_from< std::iter_value_t<I>, std::iter_reference_t<I>> constexpr auto fold_right_last( I first, S last, F f ); | (1) | (since C++23)
template< ranges::bidirectional_range R, /*indirectly-binary-right-foldable*/< ranges::range_value_t<R>, ranges::iterator_t<R>> F > requires std::constructible_from< ranges::range_value_t<R>, ranges::range_reference_t<R>> constexpr auto fold_right_last( R&& r, F f ); | (2) | (since C++23)
Helper concepts |  |
template< class F, class T, class I > concept /*indirectly-binary-left-foldable*/ = /* see description */; | (3) | (exposition only*)
template< class F, class T, class I > concept /*indirectly-binary-right-foldable*/ = /* see description */; | (4) | (exposition only*)

Right-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
`f(x1, f(x2, ...f(xn-1, xn)))`, where `x1`, `x2`, ..., `xn` are elements of the range.

Informally, `ranges::fold_right_last` behaves like std::fold_left(ranges::reverse(r), *--last, /*flipped*/(f)) (assuming the range is not empty).

The behavior is undefined if `[`first`,` last`)` is not a valid range.

1) The range is `[`first`,` last`)`. Given U as decltype(ranges::fold_right(first, last, std::iter_value_t<I>(*first), f)), equivalent to:

    if (first == last)
        return std::optional<U>();
    I tail = ranges::prev(ranges::next(first, std::move(last)));
    return std::optional<U>(std::in_place, ranges::fold_right(std::move(first), tail,
        std::iter_value_t<I>(*tail), std::move(f)));

2) Same as (1), except that uses r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.
3) Equivalent to:

Helper concepts |  |
template< class F, class T, class I, class U > concept /*indirectly-binary-left-foldable-impl*/ = std::movable<T> && std::movable<U> && std::convertible_to<T, U> && std::invocable<F&, U, std::iter_reference_t<I>> && std::assignable_from<U&, std::invoke_result_t<F&, U, std::iter_reference_t<I>>>; | (3A) | (exposition only*)
template< class F, class T, class I > concept /*indirectly-binary-left-foldable*/ = std::copy_constructible<F> && std::indirectly_readable<I> && std::invocable<F&, T, std::iter_reference_t<I>> && std::convertible_to<std::invoke_result_t<F&, T, std::iter_reference_t<I>>, std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>> && /*indirectly-binary-left-foldable-impl*/<F, T, I, std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>>; | (3B) | (exposition only*)

4) Equivalent to:

Helper concepts |  |
template< class F, class T, class I > concept /*indirectly-binary-right-foldable*/ = /*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>; | (4A) | (exposition only*)
Helper class templates |  |
template< class F > class /*flipped*/ { F f; // exposition only public: template< class T, class U > requires std::invocable<F&, U, T> std::invoke_result_t<F&, U, T> operator()( T&&, U&& ); }; | (4B) | (exposition only*)

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to fold
r | - | the range of elements to fold
f | - | the binary function object

### Return value

An object of type std::optional<U> that contains the result of right-fold of the given range over f.

If the range is empty, std::optional<U>() is returned.

### Possible implementations

    struct fold_right_last_fn
    {
        template<std::bidirectional_iterator I, std::sentinel_for<I> S,
                 /*indirectly-binary-right-foldable*/<std::iter_value_t<I>, I> F>
        requires
            std::constructible_from<std::iter_value_t<I>, std::iter_reference_t<I>>
        constexpr auto operator()(I first, S last, F f) const
        {
            using U = decltype(
                ranges::fold_right(first, last, std::iter_value_t<I>(*first), f));

            if (first == last)
                return std::optional<U>();
            I tail = ranges::prev(ranges::next(first, std::move(last)));
            return std::optional<U>(std::in_place,
                ranges::fold_right(std::move(first), tail, std::iter_value_t<I>(*tail),
                                   std::move(f)));
        }

        template<ranges::bidirectional_range R,
                 /*indirectly_binary_right_foldable*/<
                     ranges::range_value_t<R>, ranges::iterator_t<R>> F>
        requires
            std::constructible_from<ranges::range_value_t<R>, ranges::range_reference_t<R>>
        constexpr auto operator()(R&& r, F f) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::ref(f));
        }
    };

    inline constexpr fold_right_last_fn fold_right_last;

### Complexity

Exactly ranges::distance(first, last) applications of the function object f.

### Notes

The following table compares all constrained folding algorithms:

Fold function template | Starts from | Initial value | Return type
--- | --- | --- | ---
ranges::fold_left | left | init | U
ranges::fold_left_first | left | first element | std::optional<U>
ranges::fold_right | right | init | U
ranges::fold_right_last | right | last element | std::optional<U>
ranges::fold_left_with_iter | left | init | (1) ranges::in_value_result<I, U> (2) ranges::in_value_result<BR, U>, where BR is ranges::borrowed_iterator_t<R>
ranges::fold_left_first_with_iter | left | first element | (1) ranges::in_value_result<I, std::optional<U>> (2) ranges::in_value_result<BR, std::optional<U>> where BR is ranges::borrowed_iterator_t<R>

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_ranges_fold` | `202207L` | (C++23) | `std::ranges` fold algorithms

### Example

Run this code

    #include <algorithm>
    #include <functional>
    #include <iostream>
    #include <ranges>
    #include <utility>
    #include <vector>

    int main()
    {
        auto v = {1, 2, 3, 4, 5, 6, 7, 8};
        std::vector<std::string> vs {"A", "B", "C", "D"};

        auto r1 = std::ranges::fold_right_last(v.begin(), v.end(), std::plus<>()); // (1)
        std::cout << "*r1: " << *r1 << '\n';

        auto r2 = std::ranges::fold_right_last(vs, std::plus<>()); // (2)
        std::cout << "*r2: " << *r2 << '\n';

        // Use a program defined function object (lambda-expression):
        auto r3 = std::ranges::fold_right_last(v, [](int x, int y) { return x + y + 99; });
        std::cout << "*r3: " << *r3 << '\n';

        // Get the product of the std::pair::second of all pairs in the vector:
        std::vector<std::pair<char, float>> data {{'A', 3.f}, {'B', 3.5f}, {'C', 4.f}};
        auto r4 = std::ranges::fold_right_last
        (
            data | std::ranges::views::values, std::multiplies<>()
        );
        std::cout << "*r4: " << *r4 << '\n';
    }

Output:

    *r1: 36
    *r2: ABCD
    *r3: 729
    *r4: 42

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 27.6.18 Fold [alg.fold]

### See also

ranges::fold_right (C++23) | right-folds a range of elements (algorithm function object)
ranges::fold_left (C++23) | left-folds a range of elements (algorithm function object)
ranges::fold_left_first (C++23) | left-folds a range of elements using the first element as an initial value (algorithm function object)
ranges::fold_left_with_iter (C++23) | left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object)
ranges::fold_left_first_with_iter (C++23) | left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (algorithm function object)
accumulate | sums up or folds a range of elements (function template)
reduce (C++17) | similar to std::accumulate, except out of order (function template)

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